Metal powder composition including a bonding lubricant and a bonding lubricant comprising glyceryl stearate

ABSTRACT

The present invention concerns a metal powder composition for the powder metal industry, wherein the metal powder is selected from the group consisting of an atomized iron-based powder or a sponge iron powder, and a lubricant composition comprising glyceryl stearate.

FIELD OF THE INVENTION

The present invention relates to a metal powder composition for thepowder metal industry. Particularly the invention relates to aniron-based metal powder composition comprising glyceryl stearate.

BACKGROUND OF THE INVENTION

In industry the use of metal products manufactured by compacting andsintering iron-based powder compositions is becoming increasinglywidespread. A number of different products of varying shapes andthickness are being produced. One processing technique for manufacturethe products from the base powders is to charge the powder into a diecavity and compact the powder under high pressures. The obtained compactis then removed from the die cavity and sintered.

The quality requirements of the products are continuously raised, and inthis context one important factor is that the manufactured products havehigh and consistent density. Much effort is put into research to developsuch products and one field within this research concerns lubrication,which is used i.a. to avoid excessive wear on the die cavity duringcompaction. Lubrication is accomplished by spraying a liquid dispersionor solution of the lubricant onto the die cavity surface (externallubrication) or by blending a solid lubricant powder with the iron-basedpowder (internal lubrication). In some cases, both lubricationtechniques are utilized.

The use of external, die wall lubricants can reduce or eliminate theneed for an internal lubricant, but problems accompany externallubrication techniques. First, the film thickness within the die cavityhas a tendency to vary, and the lubricant dispersion is known to dripout of the die cavity during processing. Also, aqueous dispersions are asource of rust formation on the die cavity. Another problem is thatvarious external lubricant compositions are not necessarily sufficientto adequately lower ejection forces, especially at higher compactionpressures. Finally, as a technique, the die wall lubrication does notpermit high productivity in comparison with internal lubrication.

Lubrication by means of blending a solid lubricant into the iron-basedpowder composition has also disadvantages. One problem is that thelubricant generally has a density of about 1-1.2 g/cm³, as compared withthe density of the iron-based powder, which is about 7-8 g/cm³.Inclusion of the less dense lubricant in the composition lowers thegreen density of the compacted part. Second, internal lubricants aregenerally not sufficiently effective for reducing the ejection pressureswhen manufacturing parts having part heights in excess of about 2.5-5cm. Another problem is, when the particles of internal lubricant burnoff during sintering, pore spaces can be left in the compacted part,providing a source of weakness for the part. Many presently usedlubricants also have the disadvantage of requiring high energies forejecting the green compact from the die.

Another disadvantage with presently used lubricants is that they ofteninclude zinc stearate. This is due to the fact zinc stearate impartsgood flow properties to metal powder compositions including thisstearate. In reducing atmospheres, the zinc oxide remaining afterinitial decomposition of the stearate is reduced to zinc, which readilyvolatilises because of its low boiling point. Unfortunately, oncontacting the cooler parts of the furnace or the outside atmosphere,the zinc tends to condense or reoxidise. A consequence of reactions isthat the production has to be interrupted as the furnace has to becleaned regularly.

The problems associated with zinc stearate can be avoided by the use ofcompletely organic materials such as waxes. The wax most widely used inpowder metallurgy is ethylene-bisstearamide, EBS, (available under thename Acrawax™ C or Licowax™). This material has a high melting point(140° C.) but it burns off at relatively low temperatures and leaves nometallic residue. The most serious disadvantage is its poor flowbehaviour in metal powders.

The present invention is particularly directed to iron basedcompositions with internal lubrication and wherein the lubrication isprovided by a new lubricant composition including glyceryl stearate.

An additional field of application of the glyceryl stearate compositionsor mixtures according to the present invention is as binders forpulverulent additives to the metal powder which makes it possible toproduce non dusting powder mixtures that are also free from segregation.

Glyceryl stearate has been mentioned in connection with iron-based metalpowders for the PM-industry in U.S. Pat. No. 5,518,639 and the relatedU.S. Pat. No. 5,538,684 which discloses lubricant compositionscontaining a solid phase lubricant, such as graphite, molybdenumdisulfide, and polytetrafluoroethylene in combination with a liquidphase lubricant that is a binder for the solid phase lubricant. Thebinder can be chosen from various classes of compounds includingpolyethylene glycols, polyethylene glycol esters, partial esters of C₃₋₆polyhydric alcohols, polyvinyl esters, and polyvinyl pyrrolidones. Thebinder is solubilized in an organic solvent. This lubricant compositionis applied to the surface of a die cavity prior to compaction of themetal powder composition. The glyceryl stearate is thus known use as abinder in connection with external lubrication and, in contrast to thelubricant according to the present invention, it is not mixed with theiron-based powder and optional additives before the compaction.

Furthermore, the U.S. Pat. 5,432,223 discloses that glyceryl stearatecan be used as a plastiziser in polyvinylpyrrolidone, which is a bindingagent which may be used in metal powder compositions.

Another patent which mentions glyceryl stearate in connection with metalpowders is the U.S. Pat. No. 6,187,259. In this patent glyceryl stearateis mentioned together with a wide variety of other substances as anagent for providing hydrofobicity in rare earth alloy powders for theproduction of granules.

The U.S. Pat. No. 5,641,920 mentions the use of glyceryl monostearate asa plasticizer/compatibilizer in powders for injection moulding. In anarticle “Thermal extraction of binders and lubricants in sintering” byGerman in Advances in Powder Metallurgy & Particulate Materials, 1996glyceryl monostearate is also mentioned.

In the recently published WO 03/015962 glycerol monostearate ismentioned in lubricant systems in combination with different guanidinecompounds. The systems actually tested in combination with metal powdersdo however not include glyceryl stearate. No beneficial effects withguanidine compounds have been observed in connection with the presentinvention, a reason why such guanidine compounds are not included in thelubricant/binder system according to the present invention.

OBJECTS OF THE INVENTION

An object of the invention is to provide an iron-based powder metalcomposition comprising a lubricant resulting in compacts with high andconsistent densities.

A second object of the invention is to provide an iron-based powdermetal composition comprising a lubricant resulting in compacts requiringlow ejection energies.

A third object of the invention is to provide an iron-based powder metalcomposition having good flow and comprising a lubricant which is free ofzinc.

A fourth object of the invention is to provide an iron-based powdermetal composition, which is essentially free from dusting andsegregation and wherein the glyceryl stearate acts as a binder.

SUMMARY OF THE INVENTION

These objects as well as other objects that will be apparent from thedescription below have now been obtained according to the presentinvention by providing a metal powder composition comprising a lubricantand/or binder system comprising glyceryl stearate.

DETAILED DESCRIPTION OF THE INVENTION

As used in the description and the appended claims, the expression“iron-based powder” encompasses powders prepared by atomisation,preferably water atomisation. Alternatively, the powder may be based onsponge iron. The powders may be made up essentially of pure iron; ironpowder that has been pre-alloyed with other substances improving thestrength, the hardening properties, the electromagnetic properties orother desirable properties of the end products; and particles of ironmixed with particles of such alloying elements (diffusion annealedmixture or purely mechanical mixture). Examples of alloying elements arecopper, molybdenum, chromium, nickel, manganese, phosphorus, carbon inthe form of graphite, and tungsten, which are used either separately orin combination, e.g. in the form of compounds (Fe₃P and FeMo).Unexpectedly good results are obtained when the lubricants according tothe invention are used in combination with iron-based powders havinghigh compressibility. Generally, such powders have a low carbon content,preferably below 0.04% by weight. Such powders include e.g. Distaloy AE,Astaloy Mo and ASC 100.29, all of which are commercially available fromHöganäs AB, Sweden. The particles of iron based powders will have aweight average particle size in the range of above about 10 microns.Preferred are iron or pre-alloyed iron particles having a maximum weightaverage particle size up to about 350 microns; more preferably theparticles will have a weight average particle size in the range of about25-150 microns, and most preferably 40-100 microns.

A key feature of the present invention is the glyceryl stearatelubricant. Three forms of glyceryl stearate exist, namely mono-, di- ortristearate. Technical grade glyceryl monostearate which includes about20% glyceryl distearate is used in a preferred embodiment of theinvention.

It is preferred that the glyceryl stearate is used in combination withat least one additional lubricant/binder, which is preferably selectedfrom the group consisting of non-metallic fatty acid compounds, such asethylene bisstearamide, stearic acid, oleic acid, oleyl amide,stearamide and ethylene bisoleylamide and/or metal salts of fatty acids,such as zinc stearate, calcium stearate and lithium stearate. Accordingto a preferred embodiment of the invention the lubricant/binder is madeup by 5-95% by weight of the glyceryl stearate and 95-5% by weight ofthe additional lubricant/binder. The presently most preferred embodimentof the lubricant/binder is a composition comprising 5-95% by weight ofthe glyceryl stearate and 95-5% by weight of ethylene bisstearamide.Most preferably the lubricant/binder according to the present inventionincludes 15-40% by weight of glyceryl stearate and 85-60% by weight ofethylene bisstearamide. If more than 95% by weight of glyceryl stearateis used, inferior powder properties are obtained and the surfaces of thecompacted parts will become sticky. One aspect of the invention concernssuch a lubricant composition per se.

The total amount of the glyceryl stearate containing lubricant/binder inthe metal powder composition may vary between 0.1 and 2.0% by weight,preferably between 0.1 and 0.8% by weight.

The lubricant composition may be used as a physical mixture, but is mostpreferably used as a molten and subsequently solidified and micronisedpowder of solid particles. The average particle size of the lubricantparticles may vary, but is preferably in the range of 3-150 μm. If theparticle size is too large, it becomes difficult for the lubricant toleave the pore structure of the metal-powder composition duringcompaction and the lubricant may then give rise to large pores aftersintering, resulting in a compact showing impaired strength properties.

When the glyceryl stearate mixture according to the invention is used asa binder, the method of preparation of the metal powder mixture to becompacted may be performed as described in the U.S. Pat. No. 5,480,469or in the WO publication 01/17716 both of which are hereby incorporatedby reference. As described herein the binder efficiently exerts itsbinding effect when present in molten and, subsequently, solidifiedform, i.e. the homogeneous powder mixture is contacted with the binderin the molten state thereof, whereupon the binder is allowed tosolidify. According to our observations it has been found that it is notnecessary to melt the whole lubricant/binder composition according tothe present invention but that a partial melting is sufficient.

Apart from the iron-based powder and the lubricant/binder according tothe invention, the metal powder composition may contain one or moreadditives selected from the group consisting of binders, processingaids, hard phases and flow enhancing agents. The binder may be added tothe powder composition in accordance with the method described in U.S.Pat. No. 4,834,800 (which is hereby incorporated by reference).

The binder used in the metal-powder composition may consist of e.g.cellulose ester resins, hydroxyalkyl cellulose resins having 1-4 carbonatoms in the alkyl group, or thermoplastic phenolic resins.

The processing aids used in the metal-powder composition may consist oftalc, forsterite, manganese sulphide, sulphur, molybdenum disulphide,boron nitride, tellurium, selenium, barium difluoride and calciumdifluoride, which are used either separately or in combination.

The hard phases used in the metal-powder composition may consist ofcarbides of tungsten, vanadium, titanium, niobium, chromium, molybdenum,tantalum and zirconium, nitrides of aluminium, titanium, vanadium,molybdenum and chromium, Al₂O₃, B₄C, and various ceramic materials.

The flow enhancing agent may e.g. be nano-particles of silicon dioxideor other substances of the type disclosed in the U.S. Pat. No. 5,782,954which is hereby incorporated by reference.

In brief a powder compositions which is especially preferred accordingto the invention is a metal powder composition including a metal basepowder; one or more pulverulent additives, wherein the particles of atleast one of the additives are bonded to the metal base powder particlesby an at least partially molten solidified mixture consisting of 5-95%by weight of the glyceryl stearate and 95-5% by weight of at least onelubricant selected from the group consisting of non-metallic fatty acidcompounds and a metal salts of fatty acids.

Another embodiment of the invention concerns a metal powder compositionincluding a metal base powder, optionally one or more pulverulentadditives and 0.1-2.0% by weight of a pulverulent lubricant compositionincluding mixture consisting of 5-95% by weight of the glyceryl stearateand 95-5% by weight of at least one lubricant selected from the groupconsisting of non-metallic fatty acid compounds and a metal salts offatty acids.

With the aid of conventional techniques, the iron-based powder and thelubricant particles are mixed to a substantially homogeneous powdercomposition.

EXAMPLE

The following example, which is not intended to be limiting, presentcertain embodiments and advantages of the present invention. Unlessotherwise indicated any percentages are on a weight basis.

The mixtures listed in table 1 below were prepared: TABLE 1 Sample GMS*EBS** 1  0 100  2 25 75 3 50 50*Glyceryl monostearate**Etylene bisstearamide

The iron-based powder in all samples was ASC100.29 which is a wateratomised, high purity iron powder available from Höganäs AB, Sweden. Thetotal amount of lubricant was 0.8% which was admixed in the powdercomposition together with 0.5% graphite(C-uf4). The dry ingredients inthe different test mixtures i.e. the iron powder, the lubricant and thegraphite were blended to homogenous mixtures and added to a die cavitybefore compaction. The compaction operation was performed with thedifferent powder mixtures at 400, 600 and 800 MPa at ambienttemperature.

The different test mixtures were tested as regards green density (GD),sintered density (SD), ejection energy and flow and the results areshown in FIGS. 1-5, wherein

FIG. 1 discloses the green and sintered densities obtained with theinventive powder composition including glyceryl stearate in comparisonwith the same powder including the conventionally used lubricant EBS.

FIG. 2 discloses the ejection energy as a function of the green densityobtained with the inventive powder composition in comparison with thesame powder including the conventionally used lubricant EBS.

FIG. 3 discloses the spring back as a function of the green densityobtained with the inventive powder composition in comparison with thesame powder including the conventionally used lubricant EBS.

FIG. 4 discloses the flow and apparent density of the the inventivepowder composition in comparison with the same powder including theconventionally used lubricant EBS.

EXAMPLE 2

This example illustrates further advantages with the present invention.In this example EBS/GMS in different ratios were used as abinder/lubricant according to U.S. Pat. No. 5,480,469 (Storstrom, etal.)

The binder/lubricant mixtures listed in table 2 below were prepared, andthe content of EBS and GMS expressed as % of the lubricant composition;TABLE 2 Sample EBS GMS 4 100   0 5 90 10 6 75 25 7 50 50

Four different metal powder composition were prepared by homogenouslymixing ASC 100.29 with 2% of copper powder, 0.5% of graphite and 0.6% ofa lubricant/binder composition according to table 2. The compositionswere heated to 150° C. during mixing and melting of the binder/lubricantand subsequently cooled until the binder/lubricant had solidified.

Samples from the four metal powder compositions were produced by auniaxial pressing operation at 600 MPa and spring back (SB), greendensaity (GD) and green strength were measured. From the following table3 it can be concluded that a major improvement of the green strength, aswell as in green density and spring back, have been obtained for thesamples containing a binder/lubricant composition of a mixture ofEBS/GMS compared to the samples containing EBS as a solebinder/lubricant. TABLE 3 Sample 4 5 6 7 GS (MPa) 11.7 12.9 14.6 16.6 GD(g/cm3) 7.12 7.14 7.16 7.18 SB (%) 0.31 0.29 0.27 0.25

1. A metal powder composition including a metal base powder, one or morepulverulent additives, wherein the particles of at least one of theadditives are bonded to the metal base powder particles by an at leastpartially molten, solidified mixture consisting of 5-95% by weight ofthe glyceryl stearate and 95-5% by weight of at least one lubricantselected from the group consisting of non-metallic fatty acid compoundsand metal salts of fatty acids.
 2. A metal powder composition includinga metal base powder, optionally one or more pulverulent additives and0.1-2.0% by weight of a pulverulent mixture consisting of 5-95% byweight of the glyceryl stearate and 95-5% by weight of at least onelubricant selected from the group consisting of non-metallic fatty acidcompounds and metal salts of fatty acids.
 3. Powder compositionaccording to claim 1, wherein the iron-based powder is an essentiallypure iron powder, a pre-alloyed iron powder or a diffusion alloyed ironpowder.
 4. Powder composition according to claim 1, wherein theadditives are selected from the group consisting of copper, molybdenum,chromium, manganese, nickel, phosphorus and carbon in the form ofgraphite.
 5. Powder composition according to claim 1, wherein thenon-metallic fatty acid compound is selected from the group consistingof ethylene bisstearamide, stearic acid, oleic acid, oleyl amide,stearamide and ethylene bisoleylamide.
 6. Powder composition accordingto claim 1, wherein the mixture consists of 60-85% by weight of ethylenebisstearamide and 15-40% by weight of glyceryl stearate.
 7. Powdercomposition according to claim 2, wherein the mixture is present as amolten and subsequently solidified and micronised powder of glycerylstearate and ethylene bisstearamide.
 8. Powder composition according toclaim 1, wherein the metal powder also includes one or more additivesselected from the group consisting of binders, processing aids, hardphases and flow enhancing agents.
 9. Lubricant composition comprising5-95% by weight of a glyceryl stearate and 95-5% by weight of at leastone non-metallic fatty acid compound and/or a metal salt of a fattyacid.
 10. Lubricant composition according to claim 9 wherein thenon-metallic fatty acid compound is selected from the group consistingof ethylene bisstearamide, stearic acid and oleic acid, oleyl amide,stearamide and ethylene bisoleylamide.
 11. Lubricant compositionaccording to claim 9 wherein the metal salt of the fatty acid isselected from the group consisting of zinc stearate, calcium stearateand lithium stearate.
 12. Lubricant composition according to claim 9, inthe form of a molten and subsequently micronised powder.
 13. Powdercomposition according to claim 2, wherein the iron-based powder is anessentially pure iron powder, a pre-alloyed iron powder or a diffusionalloyed iron powder.
 14. Powder composition according to claim 2,wherein the additives are selected from the group consisting of copper,molybdenum, chromium, manganese, nickel, phosphorus and carbon in theform of graphite.
 15. Powder composition according to claim 3, whereinthe additives are selected from the group consisting of copper,molybdenum, chromium, manganese, nickel, phosphorus and carbon in theform of graphite.
 16. Powder composition according to claim 13, whereinthe additives are selected from the group consisting of copper,molybdenum, chromium, manganese, nickel, phosphorus and carbon in theform of graphite.
 17. Powder composition according to claim 2, whereinthe non-metallic fatty acid compound is selected from the groupconsisting of ethylene bisstearamide, stearic acid, oleic acid, oleylamide, stearamide and ethylene bisoleylamide.
 18. Powder compositionaccording to claim 2, wherein the mixture consists of 60-85% by weightof ethylene bisstearamide and 15-40% by weight of glyceryl stearate. 19.Powder composition according to claim 3, wherein the mixture is presentas a molten and subsequently solidified and micronised powder ofglyceryl stearate and ethylene bisstearamide.
 20. Powder compositionaccording to claim 2, wherein the metal powder also includes one or moreadditives selected from the group consisting of binders, processingaids, hard phases and flow enhancing agents.
 21. Lubricant compositionaccording to claim 10, in the form of a molten and subsequentlymicronised powder.
 22. Lubricant composition according to claim 11, inthe form of a molten and subsequently micronised powder.